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Which numbers have square roots? Reed College Senior Thesis Presentation Splitting Probabilities of p-adic Polynomials Division of Mathematics and Natural Sciences Spring 2003 Asher Auel Advisor: Joe Buhler
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Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

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Page 1: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which numbers have square roots?

Reed College Senior Thesis Presentation

Splitting Probabilities of p-adic Polynomials

Division of Mathematics and Natural Sciences

Spring 2003

Asher Auel

Advisor: Joe Buhler

Page 2: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation
Page 3: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation
Page 4: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation
Page 5: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation
Page 6: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation
Page 7: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation
Page 8: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which numbers have square roots?

Page 9: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which numbers have square roots?

It depends on the rules.

Page 10: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which numbers have square roots?

It depends on the rules.

For whole numbers

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, . . .

only perfect squares have square roots

0, 1, 4, 9, 16, 25, 36, 49, . . .

Page 11: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which numbers have square roots?

It depends on the rules.

For real numbers on the number line

0−5.165 . . . −1 12 π 4.12310 . . .

Page 12: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which numbers have square roots?

It depends on the rules.

For real numbers on the number line

0−5.165 . . . −1 12 π 4.12310 . . .

only non-negative numbers have square roots

0 12 π 4.12310 . . .

in particular, only −1 doesn’t have a square root.

Page 13: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

√17 = 4.12301 . . . is irrational!

Page 14: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

√17 = 4.12301 . . . is irrational!

[Theodorus] was proving to us a certain thing aboutsquare roots, I mean the side (i.e. root) of a squareof three square units and of five square units, thatthese roots are not commensurable in length with theunit length, and he went on in this way, taking all theseparate cases up to the root of seventeen square units,at which point, for some reason, he stopped.

-Plato, Theaetetus

Page 15: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

√17 = 4.12301 . . . is irrational!

But we can get close!

Page 16: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

√17 = 4.12301 . . . is irrational!

But we can get close!

x x2 − 174.1 (4.1)2 − 17 = −.19 = −2× 10−1

4.12 (4.12)2 − 17 = −.026 = −3× 10−2

4.123 (4.123)2 − 17 = −.00087 . . . = −9× 10−4

4.1 . . . (4.1 . . .)2 − 17 = ε = u× 10−n

4, 4.1, 4.12, 4.123, 4, 1230 . . . →√

17

Page 17: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

√17 = 4.12301 . . . is irrational!

A silly attempt, but why must we choose 10 over 2?

x x2 − 173 32 − 17 = −8 = −1× 23

7 72 − 17 = 32 = 1× 25

23 232 − 17 = 512 = 1× 29

279 2792 − 17 = 77824 = 19× 212

? ?2 − 17 = λ = u× 2n

3, 7, 23, 279, . . . → ?

Page 18: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

3 = 1 + 1 · 27 = 1 + 1 · 2 + 1 · 22

23 = 1 + 1 · 2 + 1 · 22 + 0 · 23 + 1 · 24

279 = 1 + 1 · 2 + 1 · 22 + 0 · 23 + 1 · 24 + 1 · 28

...

1111111101111010001111010001001...

↓√17

Page 19: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which p-adic numbers have square roots?

Page 20: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which p-adic numbers have square roots?

It depends on p.

Page 21: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

Which p-adic numbers have square roots?

It depends on p.

For general p-adic numbers,

HHH

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there are multiple square roots missing.

Page 22: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

a

b

x +ax+b has a root?2 a,b are 2-adic numbers.

Page 23: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

a

b

x +ax+b has which root?2 a,b are 2-adic numbers.

Page 24: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation

J. P. Serre, 1968

Page 25: Which numbers have square roots? Reed College Senior ...users.math.yale.edu/~auel/papers/docs/trustee3.pdf · Which numbers have square roots? Reed College Senior Thesis Presentation